Previous work by others suggests: i) improved cardiovascular function resulting from exercise training may be primarily due to adaptations in blood flow to the skeletal muscles in both normal and cardiac patients; ii) exercise training results in reductions in skeletal muscle blood flow at any given submaximal level of exercise and increases in flow to nonmuscular tissues; iii) training increases maximal blood flow to skeletal muscle; iv) nutritional flow to skeletal muscle does not increase with training; and v) skeletal muscle capillary density may be the factor that limits VO2 max. However, in each case there are conflicting reports; none of these important concepts is clearly established in the literature. The purpose of these experiments is three-fold: i) We will determine the influence of a 12-week treadmill training program on the distribution of blood flow within and among skeletal muscles during running at different speeds in rats. Distribution of cardiac output to a number of other organs will also be measured. We hypothesize that blood flow to muscles will be optimized in the trained animals so that a precise matching of blood flow to recruited muscle fibers will permit higher flows to nonmuscular tissues during exercise. ii) We will use PS products and gravimetric techniques to estimate resting and maximal capillary surface areas in skeletal muscles of trained and untrained rats. We hypothesize that in resting muscle with normal vascular tone there will be no difference between trained and untrained, but that during maximal vasodilation the trained muscle will have a greater capillary surface area available for exchange between blood and tissue and therefore a greater transport capacity. iii) We will relate whole body VO2 max to capillary densities in the skeletal muscles of trained and untrained rats. We predict a close relationship between these two variables for the specific muscles that undergo training adaptations. We believe the results of these experiments will significantly further our understanding of the influence of training on cardiovascular function.